1. Field of the Invention
This invention relates to an improved gas-fired water heating apparatus, and more particularly, to a radially fired water heating apparatus that is adaptable for either commercial water heater or boiler use. One aspect of the invention relates to a water heating apparatus comprising a plurality of vertically stacked, multiple-turn planar coils. Another aspect of the invention relates to a water heating apparatus comprising a double walled jacket that functions as a secondary heat exchanger, transferring heat by countercurrent flow from the flue gasses to incoming combustion air. Another aspect of the invention relates to a water heating apparatus that proportions combustion air and fuel independently to control operation in response to heating demand. Still another aspect of the invention relates to a water heating apparatus that is adapted for use in a condensing environment.
2. Description of Related Art
A general review of design factors and standard references to be considered in specifying commercial water heating systems is disclosed in "A Variety Of Design Factors Govern Hot Water Heater Choices", CONSULTING/SPECIFYING ENGINEER, JANUARY 1991, p. 48-51. Such factors include, by way of example, whether the system is to utilize direct or indirect heating, the anticipated load or demand, thermal efficiency, utilization temperatures, whether the system will be used in a condensing environment, water quality, corrosion, scale, and the like.
Many direct fired, commercial water heating systems are presently available. One commercially available system, disclosed in U.S. Pat. No. 4,261,299, utilizes a horizontal combustion chamber around which water flows through a double-walled shell that is wound repeatedly around the combustion chamber with spaces between each successive winding to accommodate a countercurrent flow of exhaust gasses.
Another commercially available system, disclosed in U.S. Pat. No. 4,938,204, utilizes a dual tank design. One tank contains the primary heat exchanger in which a horizontally mounted conventional burner heats water flowing through two-pass, U-bend firetubes. Exhaust gasses that exit the primary heat exchanger at 350.degree. F. to 400.degree. F. are routed to a secondary heat exchanger where they are passed countercurrent to ambient makeup water to preheat the water before entering the primary exchanger. Makeup air is preheated to over 200.degree. F. by passing it through ductwork which surrounds the exhaust gasses exiting the secondary exchanger.
Some of the newer prior art systems utilize primary exchanger sections comprising a vertically disposed, radially directed, cylindrical burner in combination with a plurality of fixed length, copper-finned tubes arranged vertically around the burner. Water flows through the tubes, which are typically connected to headers located above and below the combustion zone, either in single or double-pass configurations. In some heaters, the copper-finned tubes are intermeshed and completely surround the burner to enhance heat transfer. Difficulties have been experienced with these heaters, however, because of the limited amount of expansion or contraction that can be accommodated with the fixed tube design, and also because any condensate that may form around the horizontal fins tends to pool up on the fins, leading to corrosion and/or scaling problems and diminished thermal efficiency.
More recently, a radially fired water heating system has been introduced that utilizes horizontally disposed coils of self-baffling extruded copper tubing. This self-baffling tubing, with a generally hexagonal cross-section, is also believed likely to trap water and corrosive products on the outside of the tubing when used in a condensing environment. Most conventional designs employing coiled copper tubing use a stainless steel heat shield that is wrapped around the burner, preventing direct impingement heating on the coils. At least one design utilizes an inlet header disposed inside the circumference of the coils, making it difficult or impossible to inspect the water side of the heat exchanger section.
Another prior art condensing water heater comprises a boiler section designed as a horizontal, double-row coil of finned copper tubing surrounding a cylindrical perforated-plate flameholder in combination with an economizer designed as a single coil of finned stainless steel tubing disposed radially outward from the primary coil.
Notwithstanding the systems disclosed in the prior art, a radially fired water heater apparatus is needed that is characterized by a plurality of horizontally disposed, multiple-looped, finned coils supported and separated vertically by baffles adapted to promote efficient heat transfer to water passing through the coils; vertical inlet and outlet manifolds disposed outside the coils for easy access and inspection, the manifolds being selectively adaptable for use in either a single-pass or multiple-pass configuration; and jackets providing countercurrent heat transfer between the exhaust products exiting the primary heat exchange section and the inlet air.